1. Field of the Invention
The present invention relates to a blood-pressure measuring apparatus which includes an inflatable cuff adapted to be worn on a body portion of a living subject and measures a blood pressure of the subject according to oscillometric method, and particularly to such an apparatus which additionally includes, for improving the accuracy of blood-pressure measurement, a pulse-wave detecting device for detecting a pulse wave propagated to a downstream-side portion of the cuff as seen in a direction of flow of blood in an artery of the body portion and determines a blood pressure of the subject based on the pulse wave detected by the pulse-wave detecting device.
2. Related Art Statement
Generally, an oscillometric-type blood-pressure (xe2x80x9cBPxe2x80x9d) measuring apparatus increases a pressing pressure of an inflatable bag accommodated in a cuff adapted to be wound around a body portion of a living subject, up to a predetermined pressure at which the bag can completely stop the flow of blood in an artery of the body portion under the cuff, subsequently slowly decreases the pressing pressure of the bag while continuously detecting the pressure in the bag, extracts a pulse wave from the continuously detected pressure of the bag, specifies a pressure of the bag at a rising point where respective amplitudes of respective heartbeat-synchronous pulses of the extracted pulse wave significantly largely changes, and determines the specified pressure of the bag as a systolic BP value of the subject.
However, the above BP measuring apparatus may not measure an accurate systolic BP value because the apparatus may not specify an accurate rising point of the pulse amplitudes of the pulse wave. More specifically described, even if the pressing pressure of the cuff may be higher than the systolic BP value of the subject, the pulsation of the artery may be transmitted to an upstream-side portion of the cuff as seen in the direction of flow of blood in the artery. In particular, in the case where the cuff is worn on a certain body portion (e.g., an ankle) of the subject where it is difficult for the cuff to completely stop the blood flow in the artery, a considerably great pulse wave may be transmitted to the upstream-side portion of the cuff while the pressing pressure of the cuff is higher than the systolic BP value of the subject. Accordingly, the pulse amplitudes of the pulse wave show an unclear rising point.
To solve the above problem, it has been proposed to provide a pulse-wave detecting device inside a downstream-side portion of an inflatable bag of a cuff and determine a BP value based on pulse amplitudes detected by the pulse-wave detecting device. For example, the pulse-wave detecting device may be one which includes a pulse-wave-detecting inflatable bag which is independent of the artery-pressing inflatable bag and is provided inside the downstream-side portion of the artery-pressing bag, and a pressure sensor which detects the pressure in the pulse-wave-detecting bag. In this case, a BP value is determined based on pulse amplitudes of a pulse wave transmitted to the pulse-wave-detecting bag. While the pressing pressure of the artery-pressing bag is higher than the systolic BP value of the subject, the pulsation of the artery may be propagated to the artery-pressing bag, but is not directly propagated to the pulse-wave-detecting bag provided inside the downstream-side portion of the artery-detecting bag. Thus, the pulse amplitudes of the pulse wave detected by the pulse-wave detecting device show a clearer rising point, which leads to determining a more accurate BP value.
However, in some cases, even pulse amplitudes of a pulse wave detected by the pulse-wave detecting device do not show a clear rising point. In particular, in the case where the cuff is worn on, e.g., an ankle where it is difficult for the cuff to completely stop the blood flow in the artery, the pulse amplitudes of a pulse wave detected by the pulse-wave detecting device may not show a clear rising point. In this case, while the pressing pressure of the artery-pressing bag is higher than the systolic BP value of the subject, the pulsation of the artery is, indeed, not directly propagated to the pulse-wave-detecting bag provided inside the downstream-side portion of the artery-detecting bag. However, the pulsation may be indirectly propagated to the pulse-wave-detecting bag via the artery-pressing bag. That is, even if the pressing pressure of the artery-pressing bag may be higher than the systolic BP value of the subject, a pulse wave is produced from an artery located on an upstream side of the cuff, and this pulse wave is propagated to an upstream-side portion of the artery-pressing bag of the cuff, and this pulse wave is detected by the pulse-wave sensor via the pulse-wave-detecting bag. Therefore, a BP value determined based on pulse amplitudes of a pulse wave detected by the pulse-wave detecting device may not be accurate.
It is therefore an object of the present invention to provide a blood-pressure measuring apparatus which measures an accurate blood pressure.
The Applicants have carried out their extensive studies to achieve the above object, and found that it is possible to measure an accurate blood pressure by subtracting, from respective amplitudes of heartbeat-synchronous pulses of a pulse wave detected by a pulse-wave sensor, the influence of respective amplitudes of heartbeat-synchronous pulses of a pulse wave propagated to an inflatable bag employed for pressing an arterial vessel of a living subject, and thereby clearly showing a rising point of the former pulse amplitudes. The present invention has been developed based on this finding.
(1) According to a first feature of the present invention, there is provided a blood-pressure measuring apparatus comprising an inflatable cuff which is adapted to be wound around a body portion of a living subject and which includes a first inflatable bag which is inflatable to press an arterial vessel of the body portion and stop flow of blood in the arterial vessel; a first pulse-wave detecting device which detects a first pulse wave which is produced from the arterial vessel and is propagated to the first inflatable bag of the cuff wound around the body portion, the first pulse wave including a plurality of heartbeat-synchronous pulses; a second pulse-wave detecting device which detects a second pulse wave which is produced from the arterial vessel and is propagated to a downstream-side portion of the cuff wound around the body portion as seen in a direction in which the blood flows in the arterial vessel, the second pulse wave including a plurality of heartbeat-synchronous pulses; amplitude correcting means for correcting at least one of (a) a first group of respective amplitudes of the heartbeat-synchronous pulses of the first pulse wave detected by the first pulse-wave detecting device and (b) a second group of respective amplitudes of the heartbeat-synchronous pulses of the second pulse wave detected by the second pulse-wave detecting device, so that at least one amplitude of the first group that corresponds to at least one heartbeat-synchronous pulse of the first pulse wave that is detected by the first pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag is substantially equal to at least one amplitude of the second group that corresponds to at least one heartbeat-synchronous pulse of the second pulse wave that is detected by the second pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag; amplitude-difference determining means for determining a difference between each of the amplitudes of the first group and a corresponding one of the amplitudes of the second group after the at least one of the first and second groups is corrected by the amplitude correcting means; and blood-pressure determining means for determining a blood pressure of the subject based on the difference between the each of the amplitudes of the first group and the corresponding one of the amplitudes of the second group.
According to this feature, the amplitude correcting means corrects at least one of the first group of amplitudes provided by the first pulse-wave detecting device and the second group of amplitudes provided by the second pulse-wave detecting device, so that one or more amplitudes of the first group that correspond to one or more pulses of the first pulse wave that are detected while the flow of blood in the arterial vessel is stopped by the first inflatable bag is substantially equal to one or more amplitudes of the second group that correspond to one or more pulses of the second pulse wave that are detected while the flow of blood is stopped by the first inflatable bag, and the amplitude-difference determining means determines a difference between each of the amplitudes of the first group and a corresponding one of the amplitudes of the second group after the first and/or second groups is corrected by the amplitude correcting means. The thus obtained amplitude differences show a clear rising point, and the blood-pressure determining means can determine an accurate blood pressure value of the subject based on the change of the respective differences between the respective amplitudes of the first group and the corresponding amplitudes of the second group.
(2) According to a second feature of the present invention that includes the first feature (1), the second pulse-wave detecting device comprises a second inflatable bag which is supported by the inflatable cuff such that the second inflatable bag is located inside a downstream-side portion of the first inflatable bag as seen in the direction in which the blood flows in the arterial vessel, and which has a dimension as measured in the direction that is smaller than a dimension of the first inflatable bag as measured in the direction; and a pressure sensor which detects the second pulse wave produced from the arterial vessel and propagated to the second inflatable bag, and which produces an electric signal representing the detected second pulse wave including the heartbeat-synchronous pulses having the respective amplitudes. According to this feature, the BP measuring device can enjoy a simple construction and accordingly can be produced with ease and at low cost.
(3) According to a third feature of the present invention that includes the second feature (2), the blood-pressure measuring apparatus further comprises a preventing member which prevents the first pulse wave from being transmitted from the first inflatable bag to the second inflatable bag.
(4) According to a fourth feature of the present invention that includes the second or third feature (2) or (3), the first pulse-wave detecting device comprises a pressure sensor which detects the first pulse wave produced from the arterial vessel and propagated to the first inflatable bag, and which produces an electric signal representing the detected first pulse wave including the heartbeat-synchronous pulses having the respective amplitudes.
(5) According to a fifth feature of the present invention that includes the fourth feature (4), the blood-pressure determining means comprises means for determining, as a systolic blood pressure of the subject, a pressure in the first inflatable bag that is detected by the pressure sensor of the first pulse-wave detecting device, at a time when the respective differences between the amplitudes of the first group and the corresponding amplitudes of the second group significantly change as the pressure of the first inflatable bag changes.
(6) According to a sixth feature of the present invention that includes any one of the first to fifth features (1) to (5), the blood-pressure measuring apparatus further comprises a pressure changing device which changes a pressure in the first inflatable bag of the cuff wound around the body portion to press the arterial vessel and stop the flow of the blood in-the arterial vessel.
(7) According to a seventh feature of the present invention that includes any one of the first to sixth features (1) to (6), the blood-pressure measuring apparatus further comprises first amplitude determining means for determining the respective amplitudes of the heartbeat-synchronous pulses of the first pulse wave that are detected by the first pulse-wave detecting device while the pressure of the first inflatable bag is changed by the pressure changing device, and thereby providing the first group of amplitudes; and second amplitude determining means for determining the respective amplitudes of the heartbeat-synchronous pulses of the second pulse wave that are detected by the second pulse-wave detecting device while the pressure of the first inflatable bag is changed by the pressure changing device, and thereby providing the second group of amplitudes.
(8) According to an eighth feature of the present invention that includes any one of the first to seventh features (1) to (7), the amplitude correcting means comprises amplification-factor determining means for determining an amplification factor based on at least one amplitude of the first group that corresponds to the at least one heartbeat-synchronous pulse of the first pulse wave that is detected by the first pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag, and at least one amplitude of the second group that corresponds to the at least one heartbeat-synchronous pulse of the second pulse wave that is detected by the second pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag; and calculating means for calculating a product of the amplification factor and each of the amplitudes of the at least one of the first and second groups, and thereby correcting the at least one of the first and second groups.
(9) According to a ninth feature of the present invention that includes any one of the first to eighth features (1) to (8), the amplification-factor determining means comprises means for determining the amplification factor by dividing an average of a plurality of amplitudes of the first group that corresponds to a plurality of heartbeat-synchronous pulse of the first pulse wave that are detected by the first pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag, by an average of a plurality of amplitudes of the second group that corresponds to a plurality of heartbeat-synchronous pulse of the second pulse wave that are detected by the second pulse-wave detecting device while the flow of the blood in the arterial vessel is stopped by the first inflatable bag, and wherein the calculating means calculates a product of the amplification factor and each of the amplitudes of the second groups, and thereby correcting the second groups, and the amplitude-difference determining means determines a difference between each of the amplitudes of the first group and a corresponding one of the amplitudes of the second group that have been corrected by the amplitude correcting means.
(10) According to a tenth feature of the present invention that includes any one of the first to ninth features (1) to (9), the blood-pressure measuring apparatus further comprises a display device which displays the blood pressure of the subject determined by the blood-pressure determining means.